Method for formation of coating film

ABSTRACT

The present invention provides a method for forming a coating film, which comprises ejecting a curable coating composition from a spray gun, spray-coating the ejected composition while applying thereto an active energy beam, and heat-curing the resulting coating film, wherein the curable coating composition contains an epoxy group-containing resin (A) and a photo-induced cationic polymerization initiator (B).

TECHNICAL FIELD

The present invention relates to a method for forming a coating film.More particularly, the present invention relates to a method for forminga coating film by using a heat- and active energy beam-curable coatingcomposition, which method can give a coating film of improved fluiditywithout adversely affecting various properties of the coating film.

BACKGROUND ART

Conventional thermosetting coatings contain a fluidity-controlling agentin order to control the fluidity of coating and give a coating film ofsmooth surface and also to substantially eliminate the sagging ofcoating applied on a vertical plane. As the fluidity-controlling agent,various types are known. Generally and widely used are, for example,inorganic additives such as AEROSIL, Bentone and the like; polyamidecompounds such as Disparlon (trade name, a product of KusumotoChemicals, Ltd.) and the like; diurea compounds obtained by the reactionof a diisocyanate compound and a primary amine; and finely dividedgelled polymers.

These fluidity-controlling agents have influences on the rheology andphysical properties of coating composition and, as a result, can improvethe spraying efficiency of coating, the sagging-preventability ofcoating film, the pattern controllability of metallic pigment, etc. Onthe other hand, the fluidity-controlling agents have had problems inthat they reduce the finish appearance (e.g. luster) of coating film,the intercoat adhesion when a plurality of coatings are applied inlayers, and the water resistance of coating film.

In order to alleviate the above problems when a conventionalfluidity-controlling agent is used, coating methods were proposed whichcomprises ejecting a curable coating composition from a spray gun andspray-coating the ejected composition while applying an active energybeam thereto. In these methods, the curable coating composition has alow viscosity right after injection but has a high viscosity when coatedon a material to be coated, whereby sagging of coating from the coatedmaterial can be prevented.

For example, in JP-A-6-65523 is disclosed a coating method whichcomprises, in coating, on a material to be coated, a high-solid coatingcontaining an acrylic resin, a heat-crosslinking agent, aphotopolymerizing monomer (which has a double bond in the molecule andcan be polymerized by an electromagnetic wave), a photopolymerizationinitiator and an organic solvent, ejecting the high-solid coating from aspray gun and spray-coating the injected coating while applying a givenelectromagnetic wave to the coating.

Also, in JP-A-7-70471 is disclosed a coating method which comprisesspraying, on a material to be coated, a high-solid coating containing amacromonomer having an ethylenically unsaturated bond at one end and aphotopolymerization initiator, while applying an ultraviolet light tothe coating particles formed by spraying and flying in the air.

These methods can certainly prevent sagging. They, however, have aproblem of no applicability as a top clear for automobiles, for thefollowing reason. In the above methods, since a photo-induced radicalpolymerization reaction is utilized, the polymerization reaction ofdouble bonds is easily hinderd by the presence of oxygen; consequently,the double bonds remain in the coating film formed, which tends to allowthe film to have various defects, for example, reduced weatherabilityand yellowing.

DISCLOSURE OF THE INVENTION

The present inventors made an extensive study in order to solve theabove-mentioned problems. As a result, the present inventors found outthat the problems can be solved by using, as a coating composition, acurable coating composition containing an epoxy group-containing resinand a photo-induced cationic polymerization initiator and curing thecomposition with an active energy beam and a heat. The present inventionhas been completed based on the above finding.

According to the present invention, there is provided a method forforming a coating film, which comprises ejecting a curable coatingcomposition from a spray gun, spray-coating the ejected compositionwhile applying thereto an active energy beam, and heat-curing theresulting coating film, wherein the curable coating composition containsan epoxy group-containing resin (A) and a photo-induced cationicpolymerization initiator (B).

In the method of the present invention, a curable coating compositioncontaining an epoxy group-containing resin capable of giving rise tophoto-induced cationic polymerization and a photo-induced cationicpolymerization initiator (the composition is hereinafter referred to asthe coating composition of the present invention) is ejected from aspray gun toward a material to be coated; the ejected coatingcomposition is spray-coated on the material to be coated while an activeenergy beam is applied to the ejected coating composition; then, theresulting coating film is heat-cured to obtain a cured coating film.According to the present method, the ejected coating composition, whencoated on the material to be coated, is already cured partially by theactive energy beam applied and has an increased viscosity; therefore, nosagging of coating from the coated material takes place; the successiveheating of the formed film accelerates the curing of the film; thereby,a cured film having excellent finish appearance can be formed.

Moreover, in the present method, it is not necessary to use, in thecurable coating composition, any resin having double bonds;consequently, the coating film formed from the curable coatingcomposition can be free from various defects (for example, reducedweatherability and yellowing) caused by the undesirable double bondsremaining in the coating film; therefore, the method of the presentinvention can be applied even in top clear coating for automobiles andhas a high industrial advantage.

The method of the present invention is described below in more detail.

Epoxy Resin-Containing Resin (A)

The epoxy group-containing resin (A) used in the present invention is apolymer which has, on an average, at least about one epoxy group in themolecule but has substantially no polymerizable double bonds. Specificexamples thereof are an epoxy group-containing acrylic resin and anepoxy group-containing polyester resin.

The epoxy group-containing acrylic resin can be obtained, for example,by copolymerizing an epoxy group-containing radical-polymerizableunsaturated monomer with an acrylic monomer and, optionally, otherradical-polymerizable unsaturated monomer.

The epoxy group-containing radical-polymerizable unsaturated monomerusable in production of the epoxy group-containing acrylic resinincludes, for example, glycidyl (meth)acrylate, allyl glycidyl ether and3,4-epoxycyclohexylmethyl (meth)acrylate.

The acrylic monomer copolymerizable with the epoxy group-containingradical-polymerizable unsaturated monomer includes, for example, alkylor cycloalkyl (meth)acrylates such as methyl (meth)acrylate, ethyl(meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,lauryl (meth)acrylate, cyclohexyl (meth)acrylate and the like;hydroxyalkyl (meth)acrylates such as hydroxyethyl (meth)acrylate,hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate and the like;fluoroalkyl (meth)acrylates such as perfluorooctylethyl (meth)acrylate,perfluoroisononylethyl (meth)acrylate and the like; (meth)acrylic acid;(meth)acrylonitrile; and acrylamides such as acrylamide,N-methylolacrylamide, N-butoxymethylacrylamide and the like. The otherradical-polymerizable unsaturated monomer usable optionally includes,for example, vinyl aromatic compounds such as styrene, α-methylstyrene,vinyltoluene and the like; olefins which may contain fluorine, such asethylene, propylene, ethylene trifluoride, ethylene tetrafluoride andthe like; vinyl compounds such as vinyl chloride, vinyl acetate and thelike; carboxyl group-containing unsaturated monomers such as itaconicacid, fumaric acid, maleic acid and the like; silane compounds such asγ-(meth)acryloyloxypropyltrimethoxysilane,γ-(meth)acryloyloxypropyltriethoxysilane,γ-(meth)acryloyloxypropylmethyldimethoxysilane, vinyltrimethoxysilane,vinyltriethoxysilane, vinyltris-(β-methoxyethoxy)silane and the like;and vinyl ethers such as butyl vinyl ether, cyclohexyl vinyl ether andthe like. These monomers can be appropriately selected, combined andused so as to satisfy the properties required for the epoxygroup-containing acrylic resin formed.

The copolymerization of the above-mentioned monomers can be conducted byvarious processes which are known per se, such as solutionpolymerization process, suspension polymerization process, bulkpolymerization process, emulsion polymerization process and the like.The epoxy group-containing acrylic resin obtained can have anumber-average molecular weight of generally about 1,500-100,000,preferably about 2,000-80,000. The epoxy group-containing acrylic resincan contain, besides the epoxy group, a functional group(s) whichtake(s) part in the crosslinking reaction of the resin during itsheat-curing, such as hydroxyl group, carboxyl group, hydrolyzable silylgroup (silanol group) and/or the like.

The epoxy group-containing polyester resin can be obtained, for example,by reacting a functional group-containing polyester resin formed beforehand, with an epoxy compound having a functional group reactive with thefunctional group of the polyester resin. It can be producedspecifically, for example, by reacting a hydroxyl group-containingpolyester resin with an epoxy compound having a functional groupreactive with hydroxyl group, such as γ-glycidoxypropyltrimethoxysilane,β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane or the like. The epoxygroup-containing polyester resin can contain, besides the epoxy group, afunctional group(s) which contribute(s) to the cross-linking reaction ofthe resin during its heat-curing, such as hydroxyl group, carboxylgroup, hydrolyzable silyl group (silanol group) and/or the like.

The epoxy group-containing polyester resin can have a number-averagemolecular weight of generally about 1,000-50,000, preferably about2,000-30,000.

The epoxy group-containing acrylic resin and the epoxy group-containingpolyester resin can be used in admixture thereof, or there can be used agraft resin obtained by grafting one of them to the other. However, useof the epoxy group-containing acrylic resin is preferable.

The epoxy group content in the epoxy group-containing resin (A) used inthe present invention is not particularly restricted and can be varieddepending upon the kind of the resin used, other conditions, etc.;however, the epoxy group content is suitably in a range of generallyabout 150 to about 30,000, preferably about 200 to about 1,000 in termsof epoxy equivalents.

The epoxy resin (A) preferably contains, besides the epoxy group, acrosslinkable functional group(s) such as hydroxyl group., carboxylgroup, hydrolyzable silyl group (silanol group) and/or the like.

Photo-Induced Cationic Polymerization Initiator (B)

The photo-induced cationic polymerization initiator (B) used in thepresent coating composition is a compound which generates a cation uponirradiation with an active energy beam and allows the epoxygroup-containing resin (A) to give rise to epoxy group ring opening andcationic polymerization. It includes, for example, hexafluoroantimonatesalts, pentafluorohydroxyantimonate salts, hexafluorophosphate salts,hexafluoroarsenate salts and other photo-induced cationic polymerizationinitiators, all represented by the following formulas:

    Ar.sub.2 I.sup.+.X.sup.-                                   (I)

(wherein Ar is an aryl group, for example, a phenyl group; and X⁻ is PF₆⁻, SbF₆ ⁻ or AsF₆ ⁻),

    Ar.sub.3 S.sup.+.X.sup.-                                   (II)

(wherein Ar and X⁻ have the same definitions as given above), ##STR1##(wherein R is an alkyl group having 1-12 carbon atoms or an alkoxy grouphaving 1-12 carbon atoms; n is an integer of 0-3; and X⁻ has the samedefinition as given above), ##STR2## (wherein Y⁻ is PF₆ ⁻, SbF₆ ⁻, AsF₆⁻ or SbF₅ (OH)⁻), ##STR3## (wherein X⁻ has the same definition as givenabove), ##STR4## (wherein X⁻ has the same definition as given above),##STR5## (wherein X⁻ has the same definition as given above), ##STR6##(wherein R⁵ is an aralkyl group having 7-15 carbon atoms or an alkenylgroup having 3-9 carbon atoms; R⁶ is a hydrocarbon group having 1-7carbon atoms or a hydroxyphenyl group; R⁷ is an alkyl group having 1-5carbon atoms which may have an oxygen atom or a sulfur atom; and X⁻ hasthe same definition as given above), ##STR7## (wherein R⁸ and R⁹ areeach independently an alkyl group of 1-12 carbon atoms or an alkoxygroup having 1-12 carbon atoms), ##STR8## (wherein R⁸ and R⁹ have thesame definitions as given above), ##STR9##

Some of the above-mentioned photo-induced cationic polymerizationinitiators (B) are commercially available under the trade names of, forexample, Cyracure UVI-6970 and Cyracure UVI-6990 (products of UnionCarbide Corp. of U.S.), Irgacure 264 (a product of Ciba-Geigy Corp.) andCIT-1682 (a product of Nippon Soda Co., Ltd.). Of the above compounds,salts containing PF₆ ⁻ as an anion are preferable in view of thetoxicity and general usability.

In the coating composition of the present invention, the amount of thephoto-induced cationic polymerization initiator (B) used can be varieddepending upon the kind of the initiator, etc. but can be generally0.01-20 parts by weight, preferably 0.1-10 parts by weight per 100 partsby weight (as solid content) of the epoxy group-containing resin (A).When the amount of the photo-induced cationic polymerization initiator(B) used is less than 0.01 part by weight, the amount of the cationgenerated is small and the curing reaction by cationic polymerizationdoes not proceed sufficiently. Meanwhile, when the amount is more than20 parts by weight, the efficiency of cationic polymerization reaches asaturation point, inviting an extra cost.

Curable Coating Composition

The coating composition of the present invention basically contains theabove-mentioned epoxy group-containing resin (A) and the above-mentionedphoto-induced cationic polymerization initiator (B). The present coatingcomposition may further contain, as necessary, for example, acrosslinking agent such as melamine resin, blocked isocyanate or thelike. The present coating composition may further contain, as necessary,a heat-curing catalyst in order to accelerate the heat-curing of theepoxy group-containing resin (A). The heat-curing catalyst usable is asfollows. The catalyst effective for the crosslinking reaction betweencarboxyl group and epoxy group includes, for example, quaternary saltcatalysts such as tetraethylammonium bromide, tetrabutylammoniumbromide, tetraethylammonium chloride, tetrabutylphosphonium bromide,triphenylbenzylphosphonium chloride and the like; and amines such astriethylamine, tributylamine and the like. Of these, quaternary saltcatalysts are preferable. A mixture of the quaternary salt and about thesame equivalent of a phosphorus compound such as dibutyl phosphate orthe like is more preferable because it can impart improved storagestability to the resulting coating without impairing its curability andmoreover can prevent reduction in electrical resistance of coating (thatis, reduction in spray coatability of coating).

The catalyst effective for the crosslinking reaction of hydrolyzablesilyl group (silanol group) includes tin catalysts such as dibutyltindilaurate, dibutyltin diacetate and the like; titanium-based catalystssuch as tetrabutyl titanate and the like; and amines such astriethylamine, tributylamine and the like.

The catalyst effective for the crosslinking reaction between hydroxylgroup and isocyanate includes, for example, metal catalysts such asbismuth nitrate, lead 2-ethylhexanoate, lead benzoate, lead oleate,sodium trichlorophenolate, sodium propionate, lithium acetate, potassiumoleate, tetrabutyltin, tributyltin chloride, dibutyltin dichloride,butyltin trichloride, tin chloride, tributyltin o-phenolate, tributyltincyanate, tin octylate, tin oleate, tin oxalate, dibutyltindi(2-ethylhexylate), dibenzyltin di(2-ethylhexylate), dibutyltindilaurate, dibutyltin diisooctylmaleate, dibutyltin sulfide, dibutyltindibutoxide, dibutyltin bis(o-phenylphenolate), dibutyltinbis(acetylacetonate), di(2-ethylhexyl)tin oxide, titanium tetrachloride,dibutyltitanium dichloride, tetrabutyl titanate, butoxytitaniumtrichloride, iron trichloride, iron (III) 2-ethylhexanoate, iron (III)acetylacetonate, ferrocene, antimony trichloride, antimonypentachloride, triphenylantimony dichloride, triphenylantimony, uraniumnitrate, cadmium nitrate, cadmium diethyldithiophosphate, cobaltbenzoate, cobalt 2-ethylhexanoate, thorium nitrate, triphenylaluminum,trioctylaluminum, aluminum oleate, diphenylmercury, zinc2-ethylhexanoate, zinc naphthenate, nickelocene, hexacarbonylmolybdenum,cerium nitrate, vanadium trichloride, copper 2-ethylhexanoate, copperacetate, manganese 2-ethylhexanoate, zirconium 2-ethylhexanoate,zirconium naphthenate, triphenylarsenic, arsenic trichloride, borontrifluoride-diethyl ether complex, pyridine borane, calcium acetate,barium acetate and the like.

The catalyst effective for the crosslinking reaction between hydroxylgroup and amino group includes, for example, sulfonic acids such asp-toluene-sulfonic acid, dodecylbenzenesulfonic acid,dinonylnaphthalenedisulfonic acid and the like; phosphoric acids such asdibutyl phosphate and the like; and adducts between the above acid andepoxy compound.

The above curing catalysts can be used singly or in combination.

The amount of the crosslinking agent or the heat-curing catalyst used isnot particularly restricted and can be varied depending upon the kindthereof, the kind of functional group contained therein, etc. However,the appropriate amount of the crosslinking agent used is generally 3-100parts by weight, preferably 5-50 parts by weight per 100 parts by weight(as solid content) of the epoxy group-containing resin (A); and theappropriate amount of the heat-curing catalyst used is generally 0.05-5parts by weight, preferably 0.1-3 parts by weight per 100 parts byweight (as solid content) of the epoxy group-containing resin (A).

The coating composition of the present invention may further contain, asnecessary, a so-called dehydrating agent such as trimethyl orthoacetateor the like in order to suppress the deterioration of coating caused bythe water present in the solvent contained therein and air. The presentcoating composition may further contain, as necessary, pigmentsgenerally used in coatings, such as coloring pigment, extender pigment,rust-preventive pigment and the like.

The coating composition of the present invention may further contain, asnecessary, for example, various resins such as polyester resin, alkydresin, silicone resin, fluororesin and the like and a non-aqueousparticulate polymer in such amounts that the curing of coating film isnot substantially impaired. The present coating composition may furthercontain, as necessary, ordinary additives used in coatings such asultraviolet absorber, oxidation inhibitor, surface conditioner,antifoaming agent and the like.

The coating composition of the present invention is used ordinarily asan organic solvent type coating composition. As the solvent, there canbe used various organic solvents for coatings, for example, an aromaticor aliphatic hydrocarbon solvent, an alcohol type solvent, an ester typesolvent, a ketone type solvent and an ether type solvent. The organicsolvent usable may be the solvent per se which are used in production ofthe resin used, or may be added later as necessary. The solid content ofthe present coating composition is not particularly restricted as longas the composition can be spray-coated, but can be generally about20-90% by weight, preferably about 30-60% by weight.

Formation of Coating Film

The method for formation of coating film according to the presentinvention is carried out by, in spray-coating a coating composition ontoa material to be coated, using, as the coating composition, theabove-mentioned heat- and active energy beam-curable coatingcomposition, ejecting the composition from a spray gun, andspray-coating the injected composition onto the material while applyingan active energy beam to the ejected composition.

The spray coating can be conducted by electrostatic spray coating,non-electrostatic spray coating or the like, all known per se. Theapplication of the active energy beam can be conducted to the coatingparticles formed by spraying and present in the air and/or to thecoating adhered to the substrate, simultaneously with the adhesion. Theactive energy beam includes an ultraviolet light and an electron beam;and the source thereof includes, for example, a mercury lamp, a xenonlamp, a carbon arc, a metal halide lamp and sunlight. The dose of theactive energy beam applied can be determined depending upon thethickening tendency of coating composition and is generally set at alevel at which the coating applied on a vertical wall does not showsagging. The dose is specifically about 100-3,000 mj/m² in the case ofan ultraviolet light, and about 2-3 Mrad in the case of an electronbeam.

The coating film formed by spray coating is then heat-cured (baked).This heat-curing can completely cure the coating film which is partiallycured by the application of an active energy beam. The conditions of theheat-curing differ depending upon the coating composition used, etc.,but appropriately are generally about 110-200° C., preferably about130-150° C. for about 10-60 minutes.

Thus, the present method can form a coating film superior in finishappearance, curability, etc.

EXAMPLES

The present invention is hereinafter described more specifically byshowing Examples. In the Examples, parts and % are by weight.

Production Example 1 Production of Epoxy Group-Containing Acrylic Resin(a-1)

410 parts of xylene and 77 parts of n-butanol were fed into a 5-literglass-made flask equipped with a stirrer, a thermometer and a coolingtube, and were heated to 125° C. using an electric mantle. Thereto wasdropwise added a mixture having the following monomer composition, at aconstant rate at that temperature in 4 hours. Incidentally,azobisisobutyronitrile is a polymerization initiator.

    ______________________________________    Glycidyl methacrylate                         432 parts (30%)    n-Butyl acrylate     720 parts (50%)    Styrene              288 parts (20%)    Azobisisobutyronitrile                          72 parts    ______________________________________

The resulting mixture was subjected to aging for 30 minutes. Thereto wasdropwise added, in 2 hours, a mixture of 90 parts of xylene, 40 parts ofn-butanol and 14.4 parts of azobisisobutyronitrile, followed by agingfor 2 hours, to obtain a solution of an epoxy group-containing acrylicresin (a-1) at a final conversion of 100%.

The polymer solution obtained had a polymer solid content of 70% and aGardner viscosity of S at 25° C., and the polymer had a number-averagemolecular weight of 3,000.

Production Example 2 Production of Alicyclic Epoxy Group-ContainingAcrylic Resin (a-2)

A solution of an alicyclic epoxy group-containing acrylic resin (a-2)was obtained at a final conversion of 100% in the same manner as inExample 1 except that the monomer composition was changed to thefollowing.

    ______________________________________    3,4-Epoxycyclohexylmethyl                         432 parts (30%)    methacrylate    Styrene              288 parts (20%)    n-Butyl acrylate     720 parts (50%)    ______________________________________

The polymer solution obtained had a polymer solid content of 70% and aGardner viscosity of Q at 25° C., and the polymer had a number-averagemolecular weight of 3,000.

Production Example 3 Production of Half Ester Group-Containing AcrylicResin (a-3)

553 parts of xylene and 276 parts of 3-methoxybutyl acetate were fedinto a 5-liter glass-made flask equipped with a stirrer, a thermometerand a cooling tube, and were heated to 125° C. using an electric mantle.Thereto was dropwise added a mixture having the following monomercomposition, at a constant rate at that temperature in 4 hours.Incidentally, p-tert-butyl peroxy-2-ethylhexanoate is a polymerizationinitiator.

    ______________________________________    Methanol half ester of maleic                         288 parts (20%)    anhydride    4-Hydroxy-n-butyl acrylate                         288 parts (20%)    n-Butyl acrylate     576 parts (40%)    Styrene              288 parts (20%)    p-tert-Butyl          72 parts    peroxy-2-ethylhexanoate    ______________________________________

The resulting mixture was subjected to aging for 30 minutes. Thereto wasdropwise added, in 2 hours, a mixture of 277 parts of 3-methoxybutylacetate and 14.4 parts of p-tert-butyl peroxy-2-ethylhexanoate, followedby aging for 2 hours, to obtain a solution of a half estergroup-containing acrylic resin (a-3) at a final conversion of 98%.

The polymer solution obtained had a polymer solid content of 55% and aGardner viscosity of M at 25° C., and the polymer had a number-averagemolecular weight of 3,500 and an acid value of 86 mg KOH/g.

Production Example 4 Production of Epoxy Group- and HydroxylGroup-Containing Acrylic Resin (a-4)

A solution of an epoxy group- and hydroxyl group-containing acrylicresin (a-4) was obtained at a final conversion of 100% in the samemanner as in Example 1 except that the monomer composition was changedto the following.

    ______________________________________    Glycidyl methacrylate                         432 parts (30%)    4-Hydroxy-n-butyl acrylate                         288 parts (20%)    n-Butyl acrylate     432 parts (30%)    Styrene              288 parts (20%)    ______________________________________

The polymer solution obtained had a polymer solid content of 70% and aGardner viscosity of U at 25° C., and the polymer had a number-averagemolecular weight of 3,000.

Production Example 5 Production of Hydroxyl Group-Containing AcrylicResin (a-5)

A solution of a hydroxyl group-containing acrylic resin (a-5) wasobtained at a final conversion of 100% in the same manner as in Example1 except that the monomer composition was changed to the following.

    ______________________________________    4-Hydroxy-n-butyl acrylate                         432 parts (30%)    n-Butyl acrylate     576 parts (40%)    Styrene              432 parts (30%)    ______________________________________

The polymer solution obtained had a polymer solid content of 70% and aGardner viscosity of U at 25° C., and the polymer had a number-averagemolecular weight of 2,000.

Production Example 6 Production of Epoxy Group-, Hydroxyl Group- andHydrolyzable Alkoxysilyl Group-Containing Acrylic Resin (a-6)

A solution of an epoxy group-, hydroxyl group- and hydrolyzablealkoxysilyl group-containing acrylic resin (a-6) was obtained at a finalconversion of 100% in the same manner as in Example 1 except that themonomer composition was changed to the following.

    ______________________________________    Glycidyl methacrylate                         504 parts (35%)    4-Hydroxy-n-butyl acrylate                         216 parts (15%)    γ-Methacryloxypropyl-                         216 parts (15%)    triethoxysilane    n-Butyl acrylate     216 parts (15%)    Styrene              288 parts (20%)    ______________________________________

The polymer solution obtained had a polymer solid content of 70% and aGardner viscosity of V at 25° C., and the polymer had a number-averagemolecular weight of 2,000.

Production Example 7 Production of Coatings

Various resin solutions were prepared at compounding ratios (solidcontents) shown in Table 1 which appears later. To each solution wereadded 1 part of Tinuvin 900 (trade name, a product of Ciba-Geigy Corp.,an ultraviolet absorber) and 0.1 part of BYK-300 (trade name, a productof BYK-Chemie Japan K.K., a surface conditioner). Each of the resultingmixtures was diluted with Swasol 1000 (trade name, a product of CosmoOil Co., Ltd., a hydrocarbon solvent), followed by viscosity adjustmentto 25 seconds as measured by Ford Cup #4 at 20° C., to produce variousclear coatings to be used in the present invention.

In Table 1,

(*1) a-7: a macromonomer having a number-average molecular weight of2,500, having a methacryloyl group at one end (monomer composition:methyl methacrylate/2-hydroxyethyl methacrylate=80/20)

(*2) UVI-6990: Cyracure UVI-6990 (trade name, a product of Union CarbideCorp., a photo-induced cationic polymerization initiator having PF₆ ⁻)

D-1173: DAROCURE 1173 (trade name, a product of Ciba-Geigy JapanLimited, a photopolymerization initiator)

(*3) Cymel 202: (trade name, a product of Mitsui Cytec Ltd., a melamineresin having a resin solid content of 80%)

SBL 3175: Sumidur BL 3175 (trade name, a product of Sumitomo BayerUrethane Co., Ltd., a blocked isocyanate having a resin solid content of75%)

(*4) 1: an equimolar mixture of tetrabutylammonium bromide and monobutylphosphate

2: Dodecylbenzenesulfonic acid

3: Dibutyltin dilaurate

4: Tetrabutyl titanate

Examples 1-8 and Comparative Examples 1-3

Cationic electrocoating and intermediate coating were applied to a dullsteel plate of 0.8 mm (thickness)×300 mm×100 mm which had been subjectedto a zinc phosphate treatment. The resulting coated plate was used as abase material and subjected to the following coating test.

Maximum Sagging-Free Film Thickness

The base material was set vertically. One of the above-produced clearcoatings was ejected from a spray gun (provided at a distance of 30 cmfrom the base material) and spray-coated onto the base material bygradient coating so that the thickness of the resulting coating filmincreased gradually. In Examples 1-8 and Comparative Examples 2-3, anultraviolet light was applied to the coating which was in the air fromthe ejection to the arrival at the base material, by the use of ahigh-pressure mercury lamp (8 kw) provided at a distance of 40 cm fromthe center of the base material; however, in Comparative Example 1, noultraviolet light was applied. The thus-obtained coated plate was placedvertically in a hot-air furnace and subjected to baking at 140° C. for30 minutes. Then, the resulting plate was observed visually. As aresult, the minimum film thickness at which sagging was seen in thecoating film of the plate after baking, was taken as the maximumsagging-free film thickness of the clear coating used.

Separately, the base material was placed horizontally. Thereto wasspray-coated one of the clear coatings so that the film thickness became30μ as cured. The coated plate was subjected to baking in a hot-airfurnace at 140° C. for 30 minutes. Then, the following tests wereconducted.

Film Appearance

The surface of coating film was observed visually and evaluatedaccording to the following standard.

O: No abnormality is seen.

Δ: Slight shrinkage and/or slight fog is seen.

X: Conspicuous shrinkage and/or conspicuous fog is seen.

Curability

The surface of coating film was rubbed 10 times with a gauze impregnatedwith xylol, and then observed and evaluated according to the followingstandard.

O: The coating film surface shows no change.

Δ: Flow is clearly seen on the coating film surface.

X: The coating film surface shows swelling and tends to show whitening.

Weatherability

Measured by a QUV accelerated exposure test using an acceleratedweathering tester manufactured by Q Panel Co.

A cycle conducted under the following test conditions:

ultraviolet application 16 H/60° C.

water condensation 8 H/50° C.

was repeated for 3,000 hours (125 cycles). The coating film after thetest was evaluated according to the following standard.

O: The coating film has substantially the same luster as before test.

Δ: The coating film shows luster reduction and whitening.

X: The coating film shows luster reduction, cracking and whiteningstrikingly.

The test results are shown in Table 1.

                                      TABLE 1    __________________________________________________________________________                                               Comparative                       Examples                Examples                       1  2  3  4  5  6  7  8  1  2  3    __________________________________________________________________________    Compounding    Epoxy group-containing resin                 a-1   60                      60 60 40    (A) (*1)     a-2      60                 a-3   40 40 40                40 40 40                 a-4         60 20 70 20 70                 a-5            50    50                 a-6                        100                 a-7                                 20    Photo-induced cationic poly-                 UVI-6990                       5  5  5  5  5  5  5  5  5    merization initiator (B) (*2)                 D-1173                              5    Crosslinking agent (*3)                 Cymel 202      30 30                 SBL 3175             30 30    Heat-curing catalyst (*4)                 1     2  2  2                 2  2  2                 2              1  1                 3                    0.5                                         0.5                 4                          1    Application of ultraviolet light                       Yes                          Yes                             Yes                                Yes                                   Yes                                      Yes                                         Yes                                            Yes                                               No Yes                                                     Yes    Evaluation    Maximum sagging-free film thickness (μm)                       60 62 65 58 59 61 60 59 22 20 60    Film appearance    ◯                          ◯                             ◯                                ◯                                   ◯                                      ◯                                         ◯                                            ◯                                               ◯                                                  ◯                                                     Δ    Curability         ◯                          ◯                             ◯                                ◯                                   ◯                                      ◯                                         ◯                                            ◯                                               X  Δ                                                     Δ    Weatherability     ◯                          ◯                             ◯                                ◯                                   ◯                                      ◯                                         ◯                                            ◯                                               Δ                                                  Δ                                                     X    __________________________________________________________________________

We claim:
 1. A method for forming a coating film comprising ejecting acurable coating composition from a spray gun, exposing the ejectedcurable coating composition to an active energy beam, coating theejected and exposed curable coating composition to form a coating film,and heat curing the coating film, wherein the curable compositioncomprises an epoxy group-containing resin (A) and a photo-inducedcationic polymerization initiator (B).
 2. The method according to claim1, wherein the epoxy group-containing resin (A) has epoxy equivalents of150-3,000.
 3. The method according to claim 1, wherein the epoxygroup-containing resin (A) contains a crosslinkable functional group inaddition to the epoxy group.
 4. The method according to claim 1, whereinthe epoxy group-containing resin (A) is an epoxy group-containingacrylic resin.
 5. The method according to claim 1, wherein thephoto-induced cationic polymerization initiator (B) is selected from thegroup consisting of a hexafluoroantimonate salt, apentafluorohydroxyantimonate salt, a hexafluorophosphate salt and ahexafluoroarsenate salt.
 6. The method according to claim 1, wherein thecurable coating composition contains the photo-induced cationicpolymerization initiator (B) in an amount of 0.01-20 parts by weight per100 parts by weight as solid of the epoxy group-containing resin.
 7. Themethod according to claim 1, wherein the curable coating compositionfurther contains a cross-linking agent and/or a heat-curing catalyst. 8.The method according to claim 1, wherein the active energy beam is anultraviolet light or an electron beam.
 9. The method according to claim1, wherein the coating film formed by spray-coating is heat-cured at atemperature of 110-200° C.
 10. The coated article obtained by a methodset forth in claim 1.